The present application is concerned with an antenna apparatus supporting adjustability of an antenna beam direction.
In mobile scenarios, wireless communications systems take advantage of antennas that allow for an adaptive steering of the radiation characteristics. The antenna is able to vary its radiation characteristics according to the instantaneous situation. I.e., the main beam of radiation can be electronically aligned towards the remote station, independent of the relative orientation between both. This leads to a high signal quality and reliable transmission without any mechanical re-orientation of the antenna.
The beam forming in antenna array relies on the phase progression and amplitude distribution along the radiating aperture. The phase and amplitude applied to the antenna elements is generated by an excitation network, the so-called beam-forming network (BFN). It allows electronic variation of the phase and the amplitude of the signal to be transmitted or the signal received. Another way of generating the phase and amplitude distribution is to equip each antenna element with a dedicated signal branch, which covers the entire chain from the antenna to signal processing in baseband. For this case, the beam-forming is calculated in the digital domain. This approach provides the highest flexibility, indeed, as it offers the maximum degree of freedom. Yet, it is also the most expensive one, necessitating the largest number of hardware components as opposed to other approaches.
The intention of using a BFN is to reduce the number of parallel signal branches and, therefore, the effort of hardware. To this end, the BFN connects to the antenna array, combines the antenna signals in receive mode or distributes the transmit signal(s) amongst the antennas in transmit mode, and provides a reduced number of ports (e.g. reduction down to one port) to the following stages. Technically, there are two types of BFNs: BFNs providing a set of fixed radiation characteristics being switchable (“fixed BFN”) and BFNs providing steerable radiation characteristics (“tunable BFN”). The former necessitates only a single switching signal that determines the radiation characteristic to be generated by selecting the proper signal path of the BFN. It is therefore easy to implement, yet display the lowest flexibility. A tunable BFN comprise steerable components such as phase shifters and amplitude shifters, which are controlled via a number of control signals. It is more flexible than the former, but necessitates more effort in design, is more prone to losses, and is more sensitive to temperature variation and environmental influences.
Fixed BFNs have been well known for several decades. A systematic approach to the design of a fixed BFN is the so-called Butler-Matrix (see J. P. Shelton et al., “Multiple Beams from Linear Arrays,” IRE Transactions on Antennas and Propagation, vol. 9, no. 2, pp. 154-161, March 1961). A Butler matrix comprises a number of hybrid couplers and delay lines to generate a pre-defined set of output signals for antenna feeding. It can be considered the circuit implementation of the fast Fourier transform. The systematic design of a Butler matrix is limited to arrays with an inter-element spacing of half a wavelength. Even though other arrangements are possible, but this results in less performance and in a limited coverage. The achievable radiation characteristics are basically pre-defined and can hardly be adjusted to a given scenario. This holds also for the combination of two or more beams of a Butler matrix.
A further approach to the design of fixed BFN is based on the eigenmodes of antenna arrays (see C. Volmer et al., “An eigen-analysis of compact antenna arrays and its application to port decoupling,” IEEE Transactions on Antennas and Propagation, vol. 56, no. 2, pp. 360-370, February 2008). As a Butler matrix, the so-called eigenmode BFN comprises hybrid couplers and delay lines and can be systematically designed. While this technique can be applied to an arbitrary array, it provides only the pre-defined eigenmode radiation characteristics of the array. An eigenmode BFN technically maintain all degrees of freedom of the array, but to achieve practical radiation patterns (e.g. for mobile satellite communications) much effort is necessitated for the proper combination of the eigenmode patterns.
Tunable BFNs are used in phase array antennas. Each antenna element is fed by a signal that is individually controlled in terms of phase and amplitude. The combination of the radiation characteristics stemming from the signals of all antennas leads to the desired radiation characteristic (see R. Baggen et al., “A Compact Phased Array for SatCom Applications,” in Proc. of the 2013 IEEE International Symposium on Phased Array Systems & Technology, Waltham, Mass., USA, 15-18 Oct. 2013, pp. 232-239 and L. Krnan et al., “Reconfigurable Digitally Scanned Polarimetric L-Band Radar”, in Proc. of the 2009 IEEE Radar Conference, Pasadena, Calif., 4-8 May 2009). Such implementations suffer from high power consumption and/or low system performance, leading to a large antenna aperture and, therefore, large dimensions. They, furthermore, are sensitive to temperature variations and environmental influences; hence, much effort for calibration has to be spent.
In radar applications, arrays are often divided into sub-arrays that are controlled in amplitude and phase for mitigation of interferers (see U. Nickel, “Subarray configuration for digital beamforming with low sidelobes and adaptive interference suppression,” in Proc. of the IEEE International Radar Conference, 1995 and U. Nickel, “Array Processing for Radar: Achievements and Challenges,” International Journal of Antennas and Propagation, July 2013). Such implementations, however, usually aim at radiation characteristics pointing towards boresight. Other implementations use sub-array configurations with each sub-array connecting to a dedicated signal branch (see R. F. Rincon, “Reconfigurable L-Band Radar,” in Proc. of the 5th European Radar Conference, Amsterdam, The Netherlands, October 2008 and D. H. Sinnott et al., “THE USE OF OVERLAPPED SUBARRAY TECHNIQUES IN o SIMULTANEOUS RECEIVE BEAM LINEAR ARRAYS,” March 1984). The signals of the sub-array elements are simply summed up, without the possibility to vary the amplitude and phase of individual sub-array elements.